Intercommunicating system



Jan. 12, 1932. c. A. ANDERSON INTERCOMMUNIGATING SYSTEM Filed' Feb. 18,1929 2 Sheets-Sheet Jan. 12, 1932. c. A. ANDERSON INTERCOMMUNICATINGSYSTEM Filed Feb. 18, 1929 2 Sheets-Sheet 2 1mm? Clarence H Hndar'snnPatented Jan. 12, 1932 UNITED STATES PATENT OFFICE CLARENCE A. ANDERSON,OF CHICAGO, ILLINOIS, ASSIGNOR, BY MESNE ASSIGNMENTS,

'IO ASSOCIATED ELECTRIC LABORATORIES, 1110., OF CHICAGO, ILLINOIS, ACORPORA- TION OF DELAWARE IN TERCOMMUNICATIRTG SYSTEM:

Application filed February 18, 1929. Serial No. 340,740.

This invention relates in general to telephone systems, but is moreparticularly concerned with small intercommunicating or private branchsystems, especially of the trunk connected type; and the object of theinvention, briefly stated, is the provision of new and improved circuitarrangements which are particularly suitable for use in such smalll'elephone systems.

A particular feature of the invention relates to the improved circuitarrangements and apparatus whereby each subscriber is enabled to ac; inthe capacity of an attendant operator; that is, eachsubscriber cananswer any incoming trunk call, and can then signal and transfer thecall to any other local subscriber.

Another feature of the invention relates to the circui'; arrangementswhich prevent other local subscribers from interfering or coming in onthe trunkafter a local subscriberhas seized an idle trunk or answered anincoming call on an inter-office trunk.

Another feature of the invention relates to the manner in which thelocal subscribers are visually signalled so that they know at all timeswhich trunks are busy, which trunks arc idle, and which trunk theyshould come in on in order to answer the call.

The foregoing and other features not specifically mentioned may bedescribed more in detail by referring to the drawings comprising Figs. 1and 2, which, when Fig. 2 is placed to the right of Figfl, show by meansof the usual circuit diagrams a sufficient amount of apparatus to enablethe invention to be described and understood.

In Fig. 1 is shown the equipment individual to local subscribers lines.-A and A, one inter-communicating trunk comprising conductors 3 to 6,inclusive, two inter-oflice trunks comprising conductors 7 to 10, in-

clusive, and 11 to 14, inclusive, of which onlyv the talking conductors7 8, 11, and 12 extend to the main exchange, and a number of commonconducl'ors comprising conductors 15 to 18, inclusive.

In the upper left hand corner of Fig. 2 is shown the equipment,comprising mostly relays, which is associated with the above-mentionedtrunks at the local exchange.

In the right and lower halves of Fig. 2 is shown the equipment at themain exchange which is associated with the above-mentioned inter-officetrunks. The trunk conductors 11 and 12 terminate in the rotary lineswitch RS and in the banks of connectors such as the connector C. Thetrunk conductors 7 and 8 terminate in a rotary line switch RS and in thebanks of connectors such as connector C. The rotary line switches haveaccess to selector switches, such as S and S. The rotary line switchesmay be of the well known type in which the wipers move in a forwarddirection only and remain in the position last used. The selectors andconnectors, such as S and C, may be of the well known Strowger type ofswitch having vertical and rotary motion.

Referring more in detail to Fig. 1, each local subscribers individualequipment comprises a plurality of lamps, one for each trunk, a trunkkey, such as TK, a combined ring and transfer key, such as CK, a line orconnecting relay, a buzzer, and the substation equipment which ispreferably of the so-called monophone cradle type comprising a callingdevice, a transmitter, an induction coil, and a receiver. The trunk keysare of the locking type and when moved to the left or right remain insuch position until released by the subscriber, while the com bined ringand transfer keys are of the nonlocking type and must be held inoperated position. The base of the monophone cradle set is preferablysimilar to the cradle set disclosed in Design Patent 68,929, issuedDecember 1, 1925, to H. F. Obergfell. The lamps 23, 24, and 25 aremounted in a vertical row in the front side of the monophone base at theleft. The trunk key TK is mounted to the right and opposite the middlelamp, the normal position of the trunk key TK corresponding to themiddle lamp and the upand-down positions of the trunk key TKcorresponding to the upper and lower lamps, respectively. The ring andtransfer key is mounted to the right of the trunk key.

Having described the system and the apparatus to be used, a detaileddescription of the operation thereof will now be given. For this purposeit will be assumed that the local subscriber at substation A wishes toconverse with the local subscriber at substation A. To establish theconnection, the subscriber at substation A observes whether or not thelamp, in this case lamp 24, corresponding to the intercommunicatingtrunk, is lighted or not. In case the lamp 24 is lighted, subscriber Awill know that the intercommunieating trunk is busy and will thereforewait until the lamp becomes dark, which is an indication that such trunkis idle. In case the lamp 24 is dark the subscriber A removes hismonophone from the cradle set and thus closes a circuit for operatinghis line relay 42 as follows: from ground by way of common conductor 17,conductor 28, monophone cradle springs 44, winding of relay 42,resistance 41, normally closed springs controlled by spring 47 spring 51and its resting contact, spring 34 and its resting contact, spring 30and its resting contact, common conductor 5, and through the winding ofrelay 100 to battery. Relay 42, upon energizing, at armature 40 shortcircuits the resistance 41 so as to prevent any other local subscriberfrom seizing the intercommunicating trunk at this time; and at armatures45 and 46 connects the substation circuit to the talking conductors 3and 4 by way of the normally closed trunk key springs 35 and 36 and 31and 32. Relay 100 also energizes over the above-traced circuit and atarmature 101 completes a circuit for the intercommunicating lamp 24 asfollows: from ground by way of armature 101, common conductor 6,conductor 21, lamp 24, and to battery over common conductor 16. A branchcircuit may also be traced from grounded armature 101 through all of thelamps, such as 24, at each of the subscribers stations and back tobattery' at common conductor 16. The operation of these lamps informsthe subscribers that the intercommunicating trunk is busy.

It will be noted, that by the closure of armature 40 relay 42 shuntedout the resistance 41 and the effect of this is to prevent any otherlocal subscriber who may wish to make a call from coming in on the sametrunk. For instance, in case subscriber A should attempt connection withthe intercom municating trunk at this time, relay 42 would attempt toenergize in series with resistance 41' and relay 100, similar to the wayrelay 42 was energized. In other words, relay 42 and resistance 41"areconnected in parallel with relay 42. Under these conditions relay 42cannot pull up because of the shunting effect of relay 42, which is oflower resistance than the resistance 41 or 41'. It follows, therefore,that since relay 42 does not receive suflicient current to fullyenergize, any subscriber such as subscriber A is prevented from comingin on the intercommunicating trunk to initiate a new call, although itis possible for the subscriber A to connect with the trunk to answer thecall initiated bythe subscriber A, as will be explained presently.

After the intercommunicating lamp 24 lights up, subscriber A willoperate the combined ring and transfer key OK to the left in accordancewith the code of subscriber A. After having signalled subscriber A,subscriber A will hold the key CK in the left hand operated position.When the key CK is moved to the left handoperated position the originalenergizing circuit of relays 42 and 100 is opened at the normally closedsprings controlled by spring 47. Relay 100 deenergizes but relay 42 isheld energized over the following circuit: from grounded conductor 17 aspreviously traced, winding of relay 42, armature 40, working contact ofspring 47 and said spring, resistance 39, conductor 38, conductor 19, tobattery at common conductor 16. Relay 100, upon deenergizing, atarmature 101 removes ground from common conductor 6, therebyextinguishing the lamps 24, 24, etc. By the operation of spring 49 intoengagement with its working contact (responsive to the operation of keyOK to the left) a circuit is closed .from ground through the winding ofrelay 103, conductor 15, conductor 29, spring 49, resting contact ofsprings 37 and 33, conductor 21, lamp 24, to battery at common conductor16. Relay 103 energizes over the above-traced circuit but due to thehigh resistance of relay 103 the lamps 24, 24', etc., do not receivesuflieient energy to light brightly. However, relay 103 at armature 109short circuits itself and connects direct ground to common conductor 15.The effect of this short circuit causes relay 103 to operate in a buzzerlike manner and also causes the lamps 24, 24, etc., to flash in responseto the intermittent operation of relay 103. By the operation of spring48 into engagement with its working contact a circuit may be traced foroperating all of the buzzers 43 and 43, etc., responsive to each closureof such springs. This circuit may be traced as follows: from groundedconductor 17 conductor 28, spring 44, spring 48, buzzer 43, andconductor 19 to battery at conductor 16, and by way of conductor 20 andcommon 'conductor 18 through all of the buzzers, such as 43', etc., tobattery at common conductor 16. All the buzzers, therefore, operate inaccordance with the operation of the key GK. From the foregoing, it willbe seen that spring 47 maintains relay 42 energized, opens the beforementioned shunt circuit to permit other subscribers to come in on thistrunk line, and controls, in combination with spring 49, the flashingsignal of the intercommunicatlng lamps, while the spring 48 controls thei operation of the buzzers of all the local subscribers.

After reco nizing his code signal and observing the ashing of lamp 24',subscriber A will answer on the trunk corresponding to this lamp. Incase the trunk key TK is not already in correct position, that is, theposi tion which corresponds to the flashing lamp, subscriber A will movethe key to such a position that the key will be opposite the flashinglamp. In this instance, since the flashing lamp is 24, or the middlelamp in the vertical row, the key TK will have to be in its middle ornormal position, that position being the position shown in thedrawing-s. Subscriber A now removes his receiver and responsive theretoa circuit for energizing line relay 42' may be traced as follows: fromground by way of common conductor 17, cradle springs 44, winding ofrelay 42', resistance 41', normally closed springs controlled by spring47, springs 51, spring 34 and its resting contact, spring 30 and itsresting contact, common conductor 5 and through-the winding of relay 100to battery. Relay 42' energizesin serieswith relay 100 over this circuitbecause the previously mentioned low resistance shunt is at this timeopen at springs 47 at substation A. Relay 42', upon energizing, atarmature 40 short circuits resistance 41' and at armatures 45' and 46connects the telephone set of subscriber A to the talking conductors ofthe intercommunicating trunk by way of the heavy talking conductors andsprings 35 and 36, and 31 and 32 of the trunk key TK. The operation ofrelay 100 at this time again connects ground to common conductor 6,whereupon the lamps 24, 24, etc., burn steadily without flashing. Whensubscriber A observes that the lamp 24 has stopped flashing and is nowburning steadily he will release the key GK and permit it to return tonormal position. The circuit for relay 42 is again completed in serieswith relay 100 when the normally closed springs controlled by spring 47close responsive to the key CK returning to its normal position. Relays42 and 42 are maintained energized in parallel and in series with relay100. Subscriber A and subscriber A are now connected together and maytalk over the heavy talking conductors, the battery being furnished overconductors 3 and 4. After conversation, subscribers A and A will replacetheir receivers thereby opening the circuits of relays 42 and 42 atcradle springs 44 and 44, respectively. The circuit. of relay 100 isalso opened and accordingly said relay deenergizes and at armature 101removes ground from conductor'6, thereby extinguishing the lamps 24, 24.etc.

Having described the operation which takes place when a local subscriberdesires to call another subscriber over the intercommunicating trunk, adescription will now be given of the operation which takes place when alocal subscriber desires to complete a connection with a main exchangesubscriber. For this purpose we will assume that subscriber A desires tocomplete a connection with a main exchange subscriber. After observingthat lamp 23, for example, is dark, subscriber A will operate the trunkke, TK to the left in order to seize this trunk line. The operation ofthe key TK to the left does not perform any function at this time, butresponsive to the removal of the receiver from the cradle set relay 42is ener ized from grounded common conductor 1 by Way of conductor 28,cradle springs 44, winding of relay 42, resistance 41, normally closedsprings controlled by spring 47 springs 51, spring 34 and its restingcontact, spring 30 and its working contact, conductor 13, and throughthe lower winding of ring-up relay 104 to battery. Relays 42 and 104energize over the above-traced circuit, relay 42 performing the samefunctions as previously described and relay 104 at annature 110grounding conductor 14 to light the lamps 23, 23', etc. .By theoperation of armatures 45 and 46 by relay 42, the substation set A isbridged across the trunk line, thereby energizing line relay 117 of therotary line switch BS in the main exchange. Relay 117, upon energizing,at armature 121 grounds release trunk conductor 128 to make this trunkline busy to the connector switches having access thereto, and alsoprepares a circuit for the switching relay 116 in series with steppingmagnet 118. The operation of the rotary line switch RS now depends uponWhether the wipers 125 to 127, inclusive, are resting upon an idle orbusy trunk. Assuming that the wipers are resting on a busy trunk, testwiper 126 engages a grounded busy contact. The ground prevents theoperation of relay 116, but energizes the stepping magnet 118, and suchmagnet interrupts its own circuit and steps the wipers of the rotaryline switch to the next trunk. If this trunk is busy the magnet operatesagain and the wipers continue to rotate until the test wiper 126 engagesan idle contact, whereupon switching relay 116 energizes, since it is nolonger short circuited. Relay 116, upon energizing, at armature 120 openthe circuit of the stepping magnet 118 at its resting contact and at itsworking contact connects ground by way or armature 121 to test wiper 126for temporarily rendering this trunk busy. At armatures 119 and 124 thetalking conductors are cut through to the wipers 125 and 127, whereuponthe line and release relays in the selector S operate in the well knownmanner and the release relay transmits ground back over the releasetrunk to maintain relay 116 energized and this trunk line busy. When thetalking conductors 11 and 12 are cut through to the selector. S dialtone through the lower winding of the l1ne relay of the selector S istransmitted to the subscriber A over the heavy talking conductors in thewell known manner.

Subscriber A, upon hearing the dial tone, will operate his callingdevice CD in accordance with the directory number of the called mainexchange subscriber. The operation from now on is purely automaticoperation and being well known need not be explained further. Bothsubscriber A and the main exchange subscriber are furnished with talkingbattery from the connector switch in the main exchange. Afterconversation subscriber A replaces the monophone on the cradle, therebyseparating the springs 55 and 44, whereupon the switches in the mainexchange and line relay 42 release. It is not necessary at this time forthe subscriber to restore the key TK to its normal position because thekey in operated position does not have any eflect until the monophone isremoved from its cradle.

Having described how outgoing calls from the local exchange arecompleted to main exchange subscribers, a description will now be givenof the operation which takes place when a main exchange subscriber callsa local subscriber. For this purpose, it will be assumed that a mainexchange subscriber has operated the connector C, Fig. 2, intoengagement with the bank contacts in which the trunk conductors 11 and12 terminate. When this takes place, if the trunk line is idle, relay132 energizes in series with switching relay 116 in the well knownmanner. Due to the mechanical interlocking arrangement of the rotaryline switch the switching relay 116,.operates its armatures todisconnect the trunk conductors from the winding of line relay 17 atarmature 119 and ground at armature 124, but does not connect up theswitch wipers. Responsive to the operation of the switching relay 132 acircuit may be traced for operating the ring-up relay 104 as follows:from interrupted generator INT. GEN, at the contacts of armature 134,armature 136, wiper 139, bank contact 142, conductor 12, upper windingof relay 104, condenser C, conductor 11, bank contact 140, wiper 137,armature 135, armature 133 and its resting contact, and through theupper Winding of ring-cut-off relay 131 to battery. Relay 104 isintermittently operated responsive to the intermittent application ofgenerator, and said relay at armature 110 connects ground to commonconductor 14 for intermittently operating the trunk lamps 23, 23, etc.Any local subscriber, upon observing the flashing of a trunk lamp, suchas 23, may answer the call by first operating his trunk key such as TKto the position opposite the flashing lamp. After the operation of thetrunk key to this position the subscriber will then remove the monophonefrom its cradle.

Let it be assumed that subscriber A answers the call in the manner 'ustdescribed. Responsive to the removal of 't e monophone from its cradestand at substantion A, line relay 42 is energized in series with thelower winding of relay 104 as previously described. Relay 104 ismaintained energized over this circuit and therefore the lamps 23, 23,etc., burn steadily, which is an indication that the call has beenanswered and the trunk line is busy. By the operation of armatures 45and 46, relay 42 closes a direct current bridge across the talkingconductors 11 and 12, thereby causing the operation of the ring-cutoilrelay 131 in the connector C. Ring-cutofi relay 131 energizes and locksitself and at armatures 133 and 134 completes the talking circuit andthe circuit for the back-brid e relay 130 for -feeding battery tosubcriber The main exchange subscriber and subscriber A converse overthe heavy talking conductors as desired.

It will be assumed now that the main exchange subscriber wishes to havethe call transferred to another local subscriber. The main exchangesubscriber informs subscriber A that a connection with subscriber A, forexample, is desired. Thereupon subscriber A will operate the combinedring and transfer key OK to the right in accordance with the code ofsubscriber A and then maintain the key CK in its right hand operatedposition until the lamp 23 quits flashing and burns steadily. It will benoted that springs 52 and 53 are connected in parallel with springs 48and 49 and therefore perform the same function as such springs; that is,springs 53 controls the flashing of the lamps andspring 52 controls theoperation of the buzzers. The operation of spring 50 into engagementwith its working contact bridges impedance 54 across the talkingconductors 11 and 12 in order to maintain a direct current bridge acrosssuch conductors when relay 42 deenergizes and opens the direct currentbridge including the substation equipment at armatures 45 and 46. Atspring 51 and its resting contact the circuit of relay 42 is opened,whereupon such relay deenergizes. The circuit extending through thelower winding of relay 104 is also opened at springs 51 and accordinglyrelay 104 deenergizes and at armature 110 removes the ground fromconductor 14. This removal of ground from conductor 14 and the operationof relay 103, as previously described, causes the lamps 23, 23, etc., toflash. The circuit for energizing relay 103 is extended at this time byway of springs 53 instead of springs 49. The circuit for the buzzerincludes the common grounded conductor 17 cradle springs 44, springs 52,and common conductor 16 to battery Subscriber A, upon recognizing hiscode ring and observing which lamp is flashing, in

this case lamp 23, will operate the trunk key TK' to the'left oppositesuch lamp, and then remove his monophone from the cradle. Responsive tothe removal of the monophone from its cradle set, relays 104 and 42energize over the following circuit: from ground by Way of commonconductor 17, cradle springs 44, winding of relay 42, resistance 41,normally closed springs controlled by spring 47, spring 51', spring 34and its resting contact, spring 30 and its Working contact, conductor13, and through the lower Winding of relay 104 to batt bry. Thereoperation of relay 104 causes the lamps 23, 23, etc., to burnsteadily, which indicates to subscriber A that the subscriber A hasanswered and is connected with the interofiice trunk line. Subscriber Areleases the key CK and replaces his monophone on the cradle set.Subscriber A cannot again come in on this trunk because relay 42 willnot energize in series with resistance 41 due to the low resistanceshunt closed by armature 40 of relay 42.

The main exchange subscriber and subscriber A may converse over theheavy talking conductors as desired. In case during such conversationthe main exchange subscriber desires to talk to both subscribers A and Aat the same time, the main exchange subscriber will inform subscriber Aof such intention and subscriber A will operate the key CK to the leftin accordance with the code of subscriber A. The operation of key OK tothe left in accordance with the code causes the relay 104 to deenergize,causes relay 42' to be held in series with resistance 39 over commonconductor 16, causes the operation of the buzzers in accordance with thecode of subscriber A and causes the flashing of lamps 23, 23, etc., in amanner similar to that described for establishing an intercommunieatingconnection. Subscriber A, upon recognizing his code, and noting theflashing of lamp 23, thereupon operates his trunk key TK to the left andremoves his monophone from the cradle. Relay 42 energizes in series withrelay 104. At armature 110 relay 104 causes thelamps 23,23, etc., toburn steadily. Relay 42 is permitted at this time to energize in serieswith relay 104 because by the operation of the springs 47 the lowresistance shunt has been opened at the normally closed springscontrolled by springs 47. Relay 42 is main tained energized in parallelwith relay 42 after springs 47 are returned to normal. The subscribersmay carry on conversation over the heavy talking conductors. Afterconversation subscribers A and A replace their monophones on the cradleset, thereby opening the circuit of relays 42, 42 and 104, whereupon theapparatus is returned to its normal position and the lamps 23, 23, etc.,are rendered dark.

In case it is considered ob ect1onable to have the lamps burn steadilywhen the trunks are busy a simple modification can be readily made byadding a pair-of'normally open switch hook sprlngs similar to 44 andhave the battery conductor 26 extend through these added springs to thelamps. With this arrangement it will be obvious that the lamps will burnonly after the receiver is removed. In order to take care of incomingcalls with the above modification, a common signal would be provided toindicate an incoming call and on such a call the lamps would not seizureof said trunk, subscriber controlled.

means at each of said stations for answering said call and completingconnection between the trunk and the answering station, and meansresponsive to a subscriber answering for excluding all other subscribersfrom completing connection with said trunk.

2. In a telephone system, a plurality of subscribers stations, anincoming trunk common to said stations, a signal for said trunk at eachof said stations, means for intermittently operating all of said signalsfor indicating an incoming call responsive to the seizure of said trunk,subscriber controlled means at each of said stations for answering saidcall and completing connection between the trunk and the answeringstation, means responsive to a subscriber answering for excluding allother subscribers from completing connection with said trunk, andsubscriber controlled means for transferring such call to anothersubscribers station.

3. In a telephone system, a plurality of subscribers stations, a trunkcommon to said stations, a signal for said trunk at each of saidsubscribers stations, a key at each of said subscribers stations forpreparing a connection between the station and said trunk, meansresponsive to the removal of the receiver by a calling subscriber forclosing the prepared connection, means operative responsive there to foroperating all of said signals at said subscribers stations, means forpreventing another subscriber from seizing said trunk, and manuallyoperated means at said calling station for code signalling a desiredsubscriber, for intermittently operating all said signals, and forpermitting the called subscriber upon answering to complete connectionto said trunk.

4. In a telephone system, a plurality of subscribers stations, aplurality of trunks common to said stations a signal for each of saidtrunks at each of said subscribers stations, the unoperated condition ofsaid signals indicating idle trunks, means for operating a signal toindicate a call on a trunk, and means for operating such si nal steadilywhen the call is answered to in' icate that the trunk is busy.

5. In a telephone system, the combination of a plurality of subscribersstations, of a plurality of trunks common to said stations, a signal foreach of said trunks at each of said subscribers stations for indicatingthe idle or busy condition of said trunks, and means for flashing allthe signals associated with any particular trunk to indicate an incomingcall.

a 6. In an intercommunicating telephone system, a plurality ofsubscribers stations, a plurality of trunks common to said stations,means controlled by a calling subscriber for audibly signalling adesired called subscriber, and means for visually indicating to thecalled subscriber which trunk to answer the call on.

7 In an intercommunicating telephone system, a plurality of subscribersstations, a plurality of trunks common to said stations, meanscontrolled by a calling subscriber for audibly signalling a desiredcalled subscriber, means for visually indicating to the cal-ledsubscriber which trunk to answer the call on, and means controlled bythe response of the called subscriber for visually indicating to thecalling subscriber that the called subscriber has answered.

8. In combination with a plurality of subscribers stations, of aplurality of trunks common to said stations, a single key having adifferent position corresponding to each of said common trunks at eachsubscribers substation, and means at each subscribers station responsiveto the removal of the receiver thereat for connecting such subscribersstation to the trunk corresponding to the operated position of the keyat such station.

9. In combination with a plurality of subscribers stations, of aplurality of trunks common to said stations, a single key having adifferent position corresponding to each of said common trunks at eachsubscribers substation, means at each subscribers station responsive tothe removal of the receiver thereat for connecting such subscribersstation to the trunk corresponding to the operated position of the keyat such station, and means responsive to such connection for preventinga second subscriber from completing connection with the trunk in use.

10. In combination with a plurality of subscribers stations, of aplurality of trunks common to said stations, a single key having adifierent position corresponding to each of trunk in use.

\ 11. In a telephone system wherein a plurality of subscribers stationshaveaccess to a common trunk, a connecting relay individual to each ofsaid subscribers stations, means at a calling station for energizing oneof said relays to seize said trunk, and means controlled by said relayfor preparing a low resistance shunt circuit for the remaining relays tomake said trunk non-seizable by another station.

12. In a telephone system wherein a plurality of subscribers stationshave access to a common trunk, a connecting relay individual to each ofsaid subscribers stations, means at a calling station for energizing oneof said relays to seize said trunk, means controlled by said relay forpreparing a low resistance shunt circuit for the remaining relays tomake said trunk non-seizable by another station, and subscribercontrolled means at the calling station for opening the shunt circuit topermit another relay to ener gize to cause another subscriber to beconnected to said trunk.

13. In an intercommunicating system, a plurality of stations, aselecting key at each station, each key having a normal position and twooperated positions, a local trunk associated with the normal position ofeach key, and two main exchange trunks associated with the two operatedpositions of each key, respectively.

14. In an intercommunicating system, a plurality of stations eachincluding a telephone and a selecting key, a plurality of trunksselectable by said keys, means responsive to the removal of the receiverat any call- 1 5 ing telephone for connecting the same to the trunkselected by the associated key, means for preventing the connection ofany other telephone to said trunk, a signalling key at the callingstation for signalling 2. called station, and means dependent on thecontinued actuation of said signalling key for rendering said preventingmeans temporarily inoperative, so that the called telephone can beconnected to the trunk. I

15. In a telephone system, a plurality of subscribers stations, anincoming trunk common to said stations, a signal at each of saidstations operated responsive to the seizure of said trunk at the distantend, means at each 130 of said stations for answering said call, andmeans at the answering station for transferring the call to any other ofsaid stations.

16. In a telephone system, a plurality of subscribers stations, aplurality of incoming trunks common to said stations, a plurality ofsignals at each station, one for each trunk, means responsive to theseizure of one of said trunks at the distant end for operating thesignal associated with the seized trunk at each station, and subscribercontrolled means at each station for answering the call and fortransferring the call to any other of said stations.

17. In a telephone system, a plurality of stations, a trunk common tosaid stations, means at a calling station for connecting the station tothe trunk, means for preventing the connection of any other station tosaid trunk, a key for signalling a called station, and means dependentupon the continued operation of said key for rendering said preventingmeans inoperative to allow said called station to be connected to saidtrunk.

18. In a telephone system, a plurality of subscribers stations, aplurality of incoming trunks common to said stations, signals at each ofsaid stations, means common to all of said stations for operating saidsignals to notify the subscribers at all of said stations of an incomingcall on one of said trunks and of the particular trunk on which the callis waiting, and means at each station for answering said call.

19. In a telephone system, a plurality of subscribers stations, aplurality of incoming trunks common to said stations, signals at eachstation for notifying the subscriber thereat of an incoming call on oneof said trunks and of the particular trunk on which the call is waiting,means at each station for answering said call, and means responsive tothe answering of the call at any station for operating the signals in adistinctive manner to notify the subscribers at the remainder of thestations that the call has been answered.

20. In a telephone system, a plurality of subscribers stations, aplurality of incoming trunks common to said stations, means at eachstation for notifying the subscriber thereat of an incoming call on oneof said trunks and of the particular trunk on which the call is waiting,means at each station for answering said call, and means for preventingthe connection of any other station to said trunk.

to be answered, a plurality of other stations,

and means at said one station for transferring the connection to anyother desired one of said stations of the system.

In witness whereof, I hereunto subscribe my name this 14th day ofFebruary, A. D.

CLARENCE A. ANDERSON.

